1. Field of the Invention
The present invention relates to a monopulse antenna apparatus and antenna structure configured by using a single horn.
2. Description of the Prior Art
As a conventional tracking method used in a tracking radar apparatus, a beam switching method or a sequential lobing, and a monopulse method are known. The monopulse method is capable of detecting an angle error by using a single pulse and a four-horn monopulse method is considered to be typical.
Antenna to which the four-horn monopulse method is applied is exemplarily disclosed in Japanese Laid-open publication 59-99804, entitled "monopulse horn antenna apparatus". Japanese Laid-open publication 59-8409, entitled "monopulse antenna" also discloses an antenna of this kind. The monopulse horn antenna apparatus excites or forms, at an aperture of the antenna, a TE.sub.10 mode wave (TE stands for transverse electric) for at least a sum beam, and a TE.sub.20 mode wave for a differential mode, as described in a paragraph of "Claims of the Invention" of the specification. In the monopulse horn antenna apparatus, a plurality of horn antennas which can form monopulse beams in the H plane, are arranged in the E plane. For that purpose, a plurality of partitions made of a metallic plate, are inserted into each of these horn antennas in the direction of the H plane, thus forming monopulse beams in both the E plane and H plane.
The above-mentioned monopulse antenna is provided by improving characteristics of each primary horn in a multi-horn configuration. More specifically, the monopulse antenna has an aperture configured by four primary horns, in which a radiation-direction controlling board with a crisscross shape consisting of a metallic conductor plate, is arranged in parallel with the axis of the antenna, so as to increase the total directivity of the antenna.
Referring to FIGS. 14 to 16, a well-known monopulse antenna adopting a four-horn configuration will be described. FIG. 14 shows a structure of a conventional monopulse antenna of four-horn type, which comprises a main reflector 101, a subreflector 102, a horn 103 consisting of four horns, and a comparator (amplitude comparator) 104 for obtaining sum signals and difference signals which will be described later. FIG. 15 is a perspective view of the horn 103 which is divided or partitioned into four horns, shown as horns A, B, C and D. FIG. 16 is a block diagram of the comparator 104, showing its internal structure.
The comparator as shown in FIG. 16 has hybrid circuits 105 to 108 such as a magic T and the like, for deriving sum signals and difference signals from the four horns. It should be noted that the sum of signals is shown by .SIGMA. and the difference of signals is depicted as .DELTA.. Specifically, the comparator 104 produces a sum-signal output .SIGMA. 111, denoted by A+B+C+D, and similarly provides a sum-signal output .SIGMA. 113, given by ((A+C)-(B+D)).
The comparator 104 also produces a difference-signal output .DELTA.A.sub.Z 112 denoted by ((A+B)-(C+D)), which is an error signal with respect to the horizontal direction (in the direction associated with the angle of depression). Furthermore, a difference-signal output .DELTA.E.sub.1 110 expressed by ((A+D)-(B+C)) is provided by the comparator 104, which is an error signal with respect to the vertical direction (in the direction associated with the angle of elevation).
FIG. 17 is a structure of a higher-mode monopulse antenna using a conventional single horn, which is described, for example, in "Handbook of Antenna Engineering" edited by Institute of Telecommunications Engineers, Ohm Publications, a paragraph 9.6.3 (1980). This antenna comprises a main reflector 201, a subreflector 202, a higher-mode detector 204 connected to a single horn 203, and a reference signal detector 205.
The higher-mode detector 204 is, say, a TM.sub.10 (TM indicates transverse magnetic) mode detector and produces .DELTA.A.sub.Z +j.DELTA.E.sub.1 as an error signal 207. Note that j indicates the signal phase is shifted in 90.degree.. The reference signal detector 205 comprises, for example, a waveguide with a taper, a circular polarization/linear polarization converter, a circular waveguide (TE.sub.11)/rectangular waveguide (TE.sub.10) converter and the like, which are not shown in the figure.
However, the previously described conventional four-horn type monopulse antenna requires four independent horns or four-partitioned horns, which makes the antenna apparatus itself larger in size and brings disadvantages from a cost perspective. Even if the antenna apparatus can be miniaturized, there is a problem that leakage power from the subreflector becomes large and performance of the antenna is deteriorated accordingly.
With respect to the higher-mode monopulse antenna utilizing a conventional single horn, a higher-mode detector generally comprises a mode coupler of multi-aperture type and a combining circuit consisting of a waveguide for coupling outputs from the mode coupler. This kind of monopulse antenna also raises a problem that the antenna apparatus becomes larger in size.